The present disclosure relates to a method for rapidly cleaning a glass workpiece surface using a fixed abrasive article including cerium oxide particles dispersed in a binder.
Glass articles are extensively found in homes, offices and factories in the form of lenses, prisms, mirrors, CRT tubes, flat display glass, vehicle windshields, computer disc substrates, windows, furniture glass, art glass and the like. The grinding, finishing and polishing of these types of glass objects to an optical clarity is of utmost importance. If present, defects, imperfections, and even minute scratches can inhibit the optical clarity of the glass article. Thus, it is desired that the glass be essentially free of any defects, imperfections, scratches and be optically clear. There are three main processes or steps for finishing glass: rough grinding, fining and polishing.
The rough grinding step generates any desired curve or radius in the glass with an abrasive tool. Typically this abrasive tool includes a super-hard abrasive particle such as a diamond, tungsten carbide or cubic boron nitride. The abrasive tool in this rough grinding process imparts course scratches into the glass surface such that the resulting glass surface is neither precise enough nor smooth enough to directly polish to an optically clear state.
The fining step refines the coarse scratches generated by the rough grinding process. In general, the fining process removes the deep scratches remaining after rough grinding and provides a substantially smooth, although not polished, surface. The fining process also results in sufficient removal of the coarse scratches so that the glass surface can be polished to an optically clear surface. If the fining process does not remove all the coarse scratches, then it can be extremely difficult for the polishing step to remove these scratches to generate an optically clear surface. There is at least one fining step, typically two or more fining steps, with each subsequent fining step using an abrasive article that contains a smaller or finer abrasive particle size than the previous step.
For glass surfaces such as CRT tube glass, this fining is typically done with abrasive slurries, although coated and lapping abrasive articles can be used.
The polishing step generates an optically clear surface on the glass article. In many instances, this polishing step is done with a loose abrasive slurry. Loose abrasive slurries typically use a plurality of very fine abrasive particles (that is, less than about 10 micrometers, usually less than about 1 micrometer) dispersed in a liquid medium such as water. The loose abrasive slurry may contain other additives such as dispersants, lubricants, defoamers and the like. Loose abrasive slurries are usually the preferred means to generate the final polish because of the ability of the loose abrasive slurries to remove essentially all the remaining scratches to generate an optically clear surface that is essentially free of any defects, imperfections and/or minute scratches.
Many attempts have been made to provide a fixed abrasive article that can polish glass to an optical quality surface.
For example, U.S. Pat. No. 5,632,668 to Lindholm et al. discloses a method for polishing an optical quality surface, such as an ophthalmic lens, using abrasive composites without an abrasive slurry. Essentially all abrasive particles eroded from the abrasive composites are removed by water from the polishing interface. Erosion of abrasive particles from the abrasive composites brings a continuous supply of new abrasive particles into engagement with the surface being polished. Thus, polishing is substantially accomplished by the abrasive particles held in the binder, not the eroded abrasive particles.
U.S. Pat. No. 5,876,268 to Lamphere et al. discloses an abrasive article for reducing the surface roughness of a glass workpiece, such as a CRT screen, in a time period of less than about one minute. The abrasive article has ceria abrasive particles dispersed in a binder, which can be formed into precisely-shaped composites.
CRT face panels are currently ground and finished on large rotary hemispherical lappers, utilizing various types of abrasive slurries and pads. The final polishing step to provide optical clarity typically uses a ceria slurry on a segmented felt pad. The slurry is pumped on to the pad-glass panel interface.
Although loose abrasive slurries are widely used for polishing glass articles, many disadvantages are associated with the slurries and the various processes. These disadvantages include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive particles and to assure a uniform concentration of abrasive particles at the polishing interface, and the need for additional equipment to prepare, handle, and also recover and recycle the loose abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability which requires additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the loose abrasive slurry eventually show undesirable wear. Further, during usage, the polishing operation is usually very untidy because the loose abrasive slurry, which is usually applied as a viscous liquid to a soft pad, splatters easily and is difficult to contain.
Much less technical industrial glass is polished offhand. This process typically utilizes a felt buff wheel mounted on a backstand grinder. A ceria-based slurry or compound polish is typically used in offhand polishing. Random scratches in the glass are often removed by offhand polishing using right angle grinders having felt pads with ceria slurries or compounds. As explained above, slurry-based polishing methods exhibit significant disadvantages.
Generally, after the polishing step, the glass is cleaned. This improves the subsequent steps, when coatings such as anti-glare, anti-static, and such are applied to the glass.
The present disclosure is directed to a method of cleaning a polished glass workpiece to optical clarity using a fixed abrasive article in a semi-dry process. Generally, the process uses a textured, three-dimensional abrasive article to remove some surface scratches from a glass workpiece, such as a CRT screen, and remove any contaminants, such as oil, from the surface. The abrasive article is brought into contact with the glass surface at a desired pressure and in the presence of water, and the abrasive article and glass are moved in relation to each other under pressure. The water is believed to mediate a chemical reaction involving ceria abrasive particles and the glass, thereby leaving a haze on the glass surface. With continued motion of the abrasive article in relation to the glass workpiece, without the addition of additional water, the haze is removed.
It will be understood that the actual time necessary to polish and clean a glass workpiece will vary depending upon a number of factors, such as the polishing apparatus used, the area of the surface to be polished and cleaned, the contact pressure, and the amount of water initially present at the interface.
The textured abrasive article used in the method of the present disclosure has a backing and an abrasive coating attached to the backing, with the coating having ceria abrasive particles dispersed in a binder. In some embodiments, the abrasive coating is a plurality of abrasive composites. The binder can be formed by an addition polymerization mechanism, that is, a free-radical or cationic polymerization, of a binder precursor. Additionally, the binder precursor can be polymerized by exposure to radiant energy, along, if necessary, with an appropriate curing agent. Preferably, the binder precursor includes multi-functional acrylate resin(s), mono-functional acrylate resin(s) and mixtures thereof.
The abrasive composites can be precisely shaped or irregularly shaped. Preferably, the abrasive composites are precisely shaped. The textured, or three-dimensional abrasive article used for polishing and cleaning in accordance with this disclosure can be referred to as xe2x80x9cstructuredxe2x80x9d in the sense of the deployment of a plurality of such precisely-shaped abrasive composites. These abrasive composites, whether precisely or irregularly shaped, can be of any geometrical shape defined by a substantially distinct and discernible boundary, such as pyramidal, truncated pyramidal, and the like.
Specifically, in one aspect, the method of cleaning glass according to the present disclosure comprises contacting the glass surface with an abrasive article, which has a textured abrasive coating having a plurality of ceria abrasive particles dispersed in a binder; moving the abrasive article in relation to the glass surface in the presence of a liquid; and then moving the abrasive article in relation to the glass surface without the presence of a liquid.
In another aspect, the method of cleaning glass comprises contacting the glass surface with a textured surface of an abrasive article comprising a plurality of ceria abrasive particles dispersed in a binder such that at least a plurality of ceria surfaces are exposed; moving the abrasive article in relation to the glass surface in the presence of a liquid; allowing the liquid to substantially evaporate while continuing to move the abrasive article relative to and in contact with the glass surface; and then removing any debris formed during any of the previous steps.